1. Field of the Invention
The present invention relates to a thin film forming apparatus using laser and, more specifically, to a film forming apparatus using laser used for forming thin film having functions and to form thin films having large areas.
2. Description of the Background Art
FIG. 148 is a conventional thin film forming apparatus using laser disclosed, for example in, Japanese Patent Laying-Open No. 4-45263 which apparatus includes a chamber 1, a substrate 2, a substrate holder 3, a heater 4, a raw material target 5, a nozzle 6, an inlet window 7, a condenser lens 9, a laser unit 10, a turntable 11, an XY stage 12, a control apparatus 13, a motor 14, a plume 15 and an evacuating apparatus 17.
The operation will be described. Laser beam 16 emitted from laser unit 10 is condensed by condenser lens 9, passes through laser inlet window 7 of chamber 1, and irradiate raw material target 5 placed on turntable 11 in chamber 1. At this time, the turntable 11 can be rotated by means of motor 14. This is to make uniform laser irradiation by rotating raw material target 5 so as to prevent local generation of craters caused by sputtering of the same portion of raw material target 5.
At the portion of target 5 which is irradiated with the laser beam, plasma is generated abruptly, and in the process of cooling of the plasma in several ten ns, there are generated isolated excited atoms and ions. These groups of excited atoms and ions have the lives of at least several microseconds, which are emitted in this space to form a plume 15 which is like a candle flame. Meanwhile, a substrate 2 is placed fixed on a substrate holder 3 opposing to raw material target 5, and the excited atoms and ions in the plume 15 reach substrate 2 and are deposited thereon, forming a thin film.
In substrate holder 3, a heater 4 for heating the substrate is provided, so as to enable post annealing in which the film deposited at a low temperature is annealed at a temperature higher than the temperature for crystallization to provide a thin film of superior quality, and allowing as-deposition in which the substrate itself is held at a temperature higher than the temperature for crystallization at the time of deposition so as to form crystallized thin film at the site. In the as-deposition method, sometimes an active oxygen atmosphere is used as well. For example, as shown in the figure, a nozzle 6 for supplying gas including oxygen is provided so that the atmosphere around the substrate 2 is made an oxygen atmosphere in forming a high temperature superconductive thin film, whereby generation of oxide on substrate 2 is promoted.
In view of enlargement of the area of thin film formation, substrate holder 3 is mounted on XY stage 12, so that the position of forming the thin film can be moved. First, a control signal corresponding to an oscillation pulse of laser unit 10 is transmitted to XY stage 12 through control apparatus 13. The XY stage 12 is driven based on the control signal, and moves the position of forming the thin film on the substrate 2 at every laser pulse. Consequently, a uniform thin film can be formed on a wide area. In the conventional example, when XY stage 12 is not driven, the area of thin film formation is limited to 10 mm.times.10 mm (with the variation of film thickness distribution of .+-.10%), and when the XY stage is driven, the area can be expanded to 35 nm.times.35 nm.
However, in the semiconductor industry, formation of a uniform thin film over a wafer of 6 to 8 inches in diameter has been desired, and conventional thin film forming apparatuses using laser could not meet such demand.
FIG. 149 shows another prior art example disclosed, for example, in Japanese Patent Laying-Open No. 4-114904. Referring to the figure, 18 denotes an oxygen ion source, 19 denotes oxygen gas and 20 denotes oxygen ion beam. The process for forming a thin film in this example is the same as that of the above described prior art example. In such a thin film forming apparatus using laser, laser beam in the form of very short pulses of ten to about several ten ns is directed to the target, and the target material in the form of atoms, molecules or clusters are supplied onto the substrate only at the time of irradiation, so as to form a thin film. The excimer laser having extremely short pulse width and high energy has such advantage that (a) it allows generation of a large amount of target raw material to be deposited on the substrate so that the rate of thin film growth can be much increased, and that (b) a thin film of which composition is not very much changed from that of the raw material target can be obtained. However, the excimer laser may degrade the quality of the film due to insufficient crystallization. In order to promote crystallization of the raw material in the form of atoms, molecules or clusters deposited on substrate 2, heating of substrate 2 by a heater provided in substrate holder 3 so as to keep the substrate at a temperature higher than the temperature for crystallization has been proposed. However, if the substrate is kept at a high temperature during thin film formation, it may induce degradation of the substrate or undesirable reaction, which is inconvenient for the functional thin film from electronic or mechanic point of view. Therefore, in this prior art example, in order to reduce problems accompanying heating of the substrate, oxygen gas 19 is introduced to ion source 18 when raw material target 5 is irradiated with laser beam 16 so that substrate 2 is irradiated with the generated oxygen ion beam 20, whereby oxygen is supplied to the thin film and the temperature of crystal growth is lowered by the oxygen bombardment. Consequently, in this known example, a Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x oxide superconductive thin film can be formed at the substrate temperature of 600.degree. C.
However, the conventional thin film forming apparatus using laser has the following problems.
First, since the area of film formation which can be formed by one plume is limited, it has been impossible to form uniform thin film over a large area such as over a wafer having 6 to 8 inches in diameter required in the semiconductor industry.
In addition, there have been the problem of degradation of the substrate derived from high temperature of film formation and lower quality of the thin film caused by undesirable side reaction induced. In addition, when the film quality is to be improved by using active ion seeds, there has been possible damage of the substrate caused by ion beams, and therefore it has been difficult to improve the quality of the film.
Further, film formation parameters such as intensity of condensed beam incident on the target, condition for laser oscillation, position of the target, pressure for film formation and so on have been set initially and thereafter these parameters are not controlled. Therefore, delicate control of the film quality such as change in composition or orientation of the film which depends on composition of the surface of the target or on sudden change in energy of the particles incident on the substrate could not be done.